The carbohydrate molecules of the embryonic cell surface are thought to play a role in the cell to cell interactions during development. Previous work, by others, on mouse embryos suggests that cell differentiation is associated with changes in the carbohydrate displays on the cell surface. These include major reorganizations, as reflected by changes in core synthesis and molecular weight profiles, as well as subtle modifications of terminal residues of the carbohydrate chains. The experiments proposed here will employ a monoclonal antibody (anti-IIC3), produced against F9 embryonal carcinoma cells, which detects a cell surface carbohydrate antigen (IIC3) on mouse embryos. Immunofluorescent localization experiments have indicated that the expression of IIC3 is restricted to the cells of the embryo involved in implantation and placentation. Using this monoclonal antibody we will continue to analyze the IIC3 antigen as follows: (1) The IIC3 antigen will be immunoprecipitated and its molecular structure will be analyzed by column chromatography, SDS-polyacrylimide gel electrophoresis, and glycosidase digestion; (2) the immunoprecipitable material from F9 cells will be compared to the antigens immunoprecipitated from embryonic sources, for differences or similarities in molecular structure; (3) a continued analysis of the developmental expression of IIC3 using immunofluorescence and immunocytochemistry, on in vivo and in vitro systems; (4) an analysis of its functional role using adhesion assays; and (5) an analysis of the hormonal control of IIC3 expression during development and its potential role as a hormone receptor. These experiments will provide detailed information on a new carbohydrate antigen associated with mouse development. This information will help us understand the putative role of cell surface carbohydrates as part of the controlling mechanism of cellular differentiation.